Infrared emitters/detectors/sensors are used in many applications, for example, in detecting and discriminating the presence of specific biological, chemical substances (e.g., gases).
A conventional detector or sensor typically includes a heated element as a source of infrared emission, a filter for controlling the wavelength of emitted light, and a detector for detecting the absorption of the emitted light by a substance interacting with the emitted light. The source, referred to henceforth as an IR (infrared) emitter, typically includes a wire, filament or other infrared radiating elements. To activate, the IR emitter is heated by passing electric current through the conductive wire or filament. The current is converted to heat in the wire or filament. The infrared emission from the wire or filament is proportional to the temperature and surface area of the heated element. Often, it may be desirable to pulse the infrared emission by interrupting the electrical current periodically to modulate the surface temperature of the heated element. A spectral filter is used to selectively tailor the spectrum of the infrared emission to substantially match the absorption characteristics of the target substance to be detected.
The detector is placed facing the emitter and filter for receiving the light passed through the filter. In one example of a detector, the electrical resistance R varies as a function of its temperature T, i.e., R=f{T}. The function f{T} may be determined empirically or analytically for a particular detector. The temperature T of the detector is dependent upon how fast it cools, and the cooling rate of the detector is dependent on the optical absorption characteristics of its immediate environment. In general, different substances (e.g., gases) are known to each exhibit distinct optical absorption characteristics. The spectral filter may be selected such that the infrared source and sensor forms a tuned cavity band emitter corresponding to the absorption characteristics of the gas under study. Thus, when the targeted gas is present in the optical path between the emitter and the detector, the optical energy received by the detector is reduced, and the temperature of the detector drops, which in turn results in changing of the resistance of the detector. Thereby, the gas is detected by monitoring the resistance R of the detector.
The thermal emissions of the current emitters used in the sensors have always been associated with a black body spectrum. Although a spectral filter is used to achieve a specific spectrum of interest, the cost of the sensing device may be high and the accuracy of the device may be reduced. Furthermore, the sensors constructed as described above are multi-component systems requiring special alignment, calibration, and separate electronics for both the emitter and the detector making this sensors complex and expensive.
Another technique currently used is utilizing a diode laser as emission source. While this technique is highly sensitive and less subject to contamination and false alarms than electrochemical sensors, the units are expensive for home installation. In addition, because they depend on physical band-gaps, diode lasers can only be tuned with difficulty within a very narrow range.
Recently, photonic crystal structures, such as periodic dielectric arrays, have received much attention as optical and infrared filters with controllable narrow-band infrared absorbance. These photonic structures have been developed as transmission/reflection filters.
One type of device embodying a structure similar to a photonic crystal structure is disclosed in U.S. Pat. No. 5,973,316 . The device includes a metallic film having apertures located therein in an array arranged in a pattern so that when light is incident on the apertures, surface plasmons on the metallic film are perturbed resulting in an enhanced transmission of the light emitted from individual apertures in the array. The light transmission properties of such an apparatus are strongly dependent upon the wavelength of the light. Enhanced transmission occurs for light wavelengths in relation to the inter-aperture spacing. The aperture array is used to filter light of predetermined wavelengths traversing the apertures. The device disclosed in U.S. Pat. No. 5,973,316 is primarily used in filters, and, generally, an external light source (emitter) is still needed to generate light that impinges onto the aperture array.
U.S. Pat. No. 6,756,594 discloses a sensor engine, which is a micromachined infrared absorption emitter/sensor, for detecting the presence of specific chemical or biological species. The sensor engine includes a substrate surface having a regular array of emission features disposed thereon. The substrate is made of a metallized single-crystal silicon.
A need exists for an inexpensive emitter/detector device capable of accurately emitting and/or detecting infrared light in a specific spectrum. It is desired that the device exhibits high stability over temperature.